Simtel MSDOS - Coast to Coast/simteldosarchivecoasttocoast2.iso/asmutil/zendisk2.zip/LST14-16.ASM

Assembly Source File | 1990-02-15 | 5KB | 189 lines

;
; *** Listing 14-16 ***
;
; Demonstrates the use of a jump table to branch into
; in-line code consisting of repeated code blocks of
; varying lengths. Branches out of the in-line code with
; 1-byte displacements at both ends of the in-line code,
; for improved speed.
;
; Searches up to N bytes of a zero- terminated string for
; a character.
;
    jmp    Skip
TestString    label    byte
    db    'This is a string containing the letter '
    db    'z but not containing capital q', 0
;
; Searches a zero-terminated string for a character.
; Searches until a match is found, the terminating zero
; is found, or the specified number of characters has been
; checked.
;
; Input:
;    AL = character to search for
;    BX = maximum # of characters to search. Must be
;        less than or equal to MAX_SEARCH_LENGTH
;    DS:SI = string to search
;
; Output:
;    SI = pointer to character, or 0 if character not
;        found
;
; Registers altered: AX, BX, SI
;
; Direction flag cleared
;
; Note: Don't pass a string starting at offset 0, since a
;    match there couldn't be distinguished from a failure
;    to match.
;
MAX_SEARCH_LENGTH equ    80    ;longest supported search
                ; length
;
; Macro to create SearchTable entries.
;
MAKE_CHECK_CHAR_LABEL    macro    NUMBER
    dw    CheckChar&NUMBER&
    endm
;
; Macro to create in-line code to search 1 character.
; Gives the code block a unique label according to NUMBER.
; Each conditional branch uses the shortest possible jump
; sequence to reach NoMatch and MatchFound.
;
CHECK_CHAR    macro    NUMBER
    local    CheckMatch, Continue
CheckChar&NUMBER&:
    lodsb        ;get the character
    and    al,al    ;done if terminating zero
;
; Assemble a single conditional jump if it'll reach, or
; a conditional jump around an unconditional jump if the
; 1-byte displacement of a conditional jump won't reach.
;
if ($+2-NoMatch) le 128
    jz    NoMatch
else
    jnz    CheckMatch
    jmp    NoMatch
endif
CheckMatch:
    cmp    ah,al    ;done if matches search character
;
; Again, assemble shortest possible jump sequence.
;
if ($+2-MatchFound) le 128
    jz    MatchFound
else
    jnz    Continue
    jmp    MatchFound
endif
Continue:
    endm
;
; Macro to create in-line code to search 1 character.
; Gives the code block a unique label according to NUMBER.
; All branches use a 1-byte displacement to branch to
; NoMatch2 and MatchFound2.
;
CHECK_CHAR2    macro    NUMBER
CheckChar&NUMBER&:
    lodsb        ;get the character
    and    al,al    ;done if terminating zero
    jz    NoMatch2
    cmp    ah,al    ;done if matches search character
    jz    MatchFound2
    endm
;
; Table of in-line code entry points for maximum search
; lengths of 0 through 80.
;
SearchTable    label    word
    dw    NoMatch        ;we never match on a
                ; maximum length of 0
BLOCK_NUMBER=MAX_SEARCH_LENGTH-1
    rept    MAX_SEARCH_LENGTH
    MAKE_CHECK_CHAR_LABEL    %BLOCK_NUMBER
BLOCK_NUMBER=BLOCK_NUMBER-1
    endm
;
SearchNBytes    proc    near
    mov    ah,al        ;we'll need AL for LODSB
    cmp    bx,MAX_SEARCH_LENGTH
    ja    NoMatch        ;if the maximum length's
                ; too long for the in-line
                ; code, return a no-match
                ; status
    shl    bx,1        ;*2 to look up in word-sized
                ; table
    jmp    [SearchTable+bx] ;branch into the in-line
                ; code to do the search
;
; No match was found.
;
NoMatch:
    sub    si,si        ;return no-match status
    ret
;
; A match was found.
;
MatchFound:
    dec    si        ;point back to matching
                ; location
    ret
;
; This is the in-line code that actually does the search.
; Each repetition is uniquely labelled, with labels
; CheckChar0 through CheckChar79.
;
BLOCK_NUMBER=0
;
; These in-line code blocks use 1-byte displacements
; whenever possible to branch backward; otherwise 2-byte
; displacements are used to branch backwards, with
; conditional jumps around unconditional jumps.
;
    rept    MAX_SEARCH_LENGTH-14
    CHECK_CHAR    %BLOCK_NUMBER
BLOCK_NUMBER=BLOCK_NUMBER+1
    endm
;
; These in-line code blocks use 1-byte displacements to
; branch forward.
;
    rept    14
    CHECK_CHAR2    %BLOCK_NUMBER
BLOCK_NUMBER=BLOCK_NUMBER+1
    endm
;
; If we make it here, we haven't found the character.
;
NoMatch2:
    sub    si,si    ;return no-match status
    ret
;
; A match was found.
;
MatchFound2:
    dec    si        ;point back to matching
                ; location
    ret
SearchNBytes    endp
;
Skip:
    call    ZTimerOn
    mov    al,'Q'
    mov    bx,20            ;search up to the
    mov    si,offset TestString    ; first 20 bytes of
    call    SearchNBytes        ; TestString for 'Q'
    mov    al,'z'
    mov    bx,80            ;search up to the
    mov    si,offset TestString    ; first 80 bytes of
    call    SearchNBytes        ; TestString for 'z'
    mov    al,'a'
    mov    bx,10            ;search up to the
    mov    si,offset TestString    ; first 10 bytes of
    call    SearchNBytes        ; TestString for 'a'
    call    ZTimerOff